Galvanic cells that measure pH are well known in the prior art. Conventionally, pH sensors often consist of a measurement or pH electrode and reference electrode, each having a silver wire (Ag) with a silver chloride (AgCl) coating at its end. The pH electrode typically has an internal-filled chloride buffer in aqueous solution having a selected pH (chloride buffer) that is often a pH of about 7 and a pH sensitive glass surrounding the internal silver wire and chloride buffer. The reference electrode typically has a container with an internal-filled reference solution of potassium chloride in aqueous solution (reference solution).
The pH-sensitive glass bulb normally encloses and contacts the internal chloride buffer and is then placed in an external liquid sample or process stream to measure pH. The glass typically has two hydrated gel layers, one layer on the inside surface and another on the outside surface. The actual pH sensing is accomplished when a potential difference develops between the two hydrated gel layers. A hydrogen ion does not exist by itself in aqueous solution. It is associated with a water molecule to form a hydronium ion (H30′). The glass enclosed pH electrode develops a potential when hydronium ions get close enough to the glass bulb surface for hydrogen ions to jump and become associated with hydronium ions in an outer hydrated gel layer disposed on the glass bulb surface. This thin gel layer is essential for electrode response. The input to the pH measurement circuit in a pH sensor is the potential difference that develops between the eternal glass surface having potential Eg that is exposed to the sample liquid and the internal glass surface having potential Er that is wetted by the chloride buffer having the selected pH. The potential difference that develops follows the Nernst equation. Assuming the chloride buffer has a temperature of 25° C. and a pH of 7 then the potential difference (which is conventionally also the input to the pH measurement circuit) is:Eg−Er=0.1984 (T+273.16) (7−pH).The potential difference that develops is proportional to the deviation of the process pH from 7 pH at 25° C. If the pH of the process stream equals 7 then the potential difference measured will be zero.
However, all pH sensors have inherent sources of error. Error sources in sensors such as those described in U.S. Pat. Nos. 5,268,852 and 5,469,070 include glass impedance and liquid junction impedance. In general, solution ground is often a term used in instrumentation for glass bulb pH sensors. One potential use of a solution ground is ensuring that ground currents bypass the higher resistance path through the reference electrode to the instrument circuit ground. Another important use is in measuring glass and liquid junction impedance useful to estimate aging of the pH sensor as well as some physical conditions of the sensor. Increased sensor impedance from aging is associated with measurement errors as well as useful in predicting future sensor failure. Solution grounds are useful in sensor diagnostics because they can supply an extra electrode for injecting test currents into the sensor for diagnostic purposes.
In the prior art, a solution ground can be an external metal surface in contact with the liquid sample solution and electrically coupled to a circuit common of an instrument. These prior art solution grounds are external because they are external and normally attached to the sensor housing. Disadvantages of external solution grounds include relative high expense and difficulty in manufacturing. Also, the external solution grounds are usually made of metals such as stainless steel, platinum or titanium, which often do not establish stable potentials in the liquid sample.
Another disadvantage of external solution grounds is that they can leak voltage into the liquid sample in which pH is being measured. The leaked voltage can lead to ground loop currents, especially through the reference electrode. Ground loops can lead to damage in the reference electrode resulting in erroneous readings, calibration errors and shortened sensor life.
Finally, when a multi-sensing instrument monitors a pH sensor and one or more secondary sensors in a single sample solution, other ground loop problems and complications can result. Ground loops are especially a concern when pH sensor circuitry and secondary sensor circuitry are not isolated within the instrument. Ground loop problems are especially evident when an instrument monitors both a pH sensor and a conductivity sensor having non-isolated circuits within the multi-sensing instrument.
There exists a need for a solution ground for primary use with a pH sensor that overcomes one, some or all of the disadvantages of prior art solution grounds.